1. Field of the Invention
This invention relates to a process for modifying the crystallization temperature and other properties of polymers, especially polyamides, by freeze-grinding said polymers.
2. Prior Art
X-ray diffraction studies of synthetic linear polyamides such as nylon 6 have shown that they may exist in one or more of several different crystalline forms. Structures which have been recognized include, in the case of nylon 6, the amorphous form, the pseudo hexagonal gamma-form, and the monoclinic alpha form.
The amorphous form of nylon 6 is obtained by rapid quenching of molten polymer to a temperature below the glass transition temperature of the nylon. Both the amorphous and gamma-forms are relatively unstable upon application of heat and moisture. Heating amorphous material to a temperature between approximately 55 C and 150 C results in at least a partial conversion of the amorphous form into the gamma-form. At temperatures above 150 C, a transition of the gamma- into the alhpa-form occurs. This monoclinic alpha structure represents a highly ordered crystalline from that is stable at temperatures up to the melting point of the nylon 6. It is the most desirable crystalline form from the standpoint of obtaining the optimum physical properties with nylon 6, including mold shrinkage and maximum dimensional stability.
The "super" or morphological structure in which the crystalline units are arranged also affects the physical properties of nylons. The crystalline units are arranged in polycrystalline aggregates known as spherulites. These spherulites may be detected by microscopic examination under polarized light. They are characterized by a more or less symmetrical growth in all directions from a nucleus and are composite structures made up of crystalline and amorphous regions. The number and size of the spherulites determines the texture or graininess in the bulk of the material and influences optical as well as physical properties. Physical properties improve with increasing homogeneity and fineness of the spherulitic structure throughout the bulk of the material.
To obtain optimum physical properties in articles fabricated from nylon 6, it is desirable, therefore, to produce a highly crystalline material, crystallized predominantly in the stable alpha-form, with an extremely fine, dense and uniform morphological structure. Among the physical properties affected by increased crystallinity and improved morphological structure are abrasion resistance, heat distortion temperature, inherent stability or resistance to deformation, resistance to hot water, coefficient of expansion, hardness, tensile yield strength and surface hardness.
Customary fabricating procedures used with nylon 6 such as injection molding, which include a rapid cooling from the melt, generally result in articles which contain the different crystalline structural forms to a varying degree depending upon the thermal history of the article.
It is known that a greater degree of crystallinity is obtained when polyamides are cooled extremely slowly from the melt; however, under these conditions, large spherulites develop which is not desirable. Moreover, the process is not economical. Crystallinity and the uniformity of the morphological structure can also be increased by annealing treatments after solidification. However, such practices are not economically feasible in ordinary industrial fabricating procedures as, for example, injection molding. More recently, investigators have found that bodies shaped from polyamides having a homogeneous and fine spherulitic structure can be obtained by addition to the polyamide melt of finely divided nucleating agents which act as crystallization nuclei. (e.g. U.S. Pat. Nos. 2,855,377; 3,261,800 and 3,080,345; Belgian Patent 674,170; Japanese Patent Nos. 52-105958 and 80-041623, U.S. Pat. No. 4,501,844; Gurato, G. et al. "Influence of Nucleating Agents on the Crystallization of Nylon 6", Makromol. Chem., Vol. 179(1), pp. 231 to 245 (1978); Chatterjee, A.M., et al., "Heterogeneous Nucleation of Crystallization of High Polymers From the Melt", J. Polym. Sci., Polym. Phys. Ed., Vol. 13(12), pp. 2368-83 (1975); and Koutsky, J.A., "Nucleation of Polymer Droplets", J. Appl. Phys., Vol. 38(4), pp. 1832-39 (1967).
The function of nucleating agents when cooling semi-crystalline polymers from the molten into the solid form is to increase the number of nuclei formed in a given time interval at a predetermined temperature. The final and over-all crystallinity, however, depends not only on the number of nuclei that are formed but also on the spherulitic growth rate from such nuclei. As noted above, spherulites develop with respect to a center, or nucleus, of growth. Addition of the nucleating agents thus provides a large number of sites for growth upon cooling from a melt. In order to be of practical use, such nucleating agents not only must produce a fine spherulitic structure but also must do this under conditions of rapid cooling to a temperature above the glass transition temperature of the polyamide, i.e., they must reduce the time that is necessary under a given set of conditions for crystallization to start. This time is usually referred to as "induction time". Subsequent growth from the spherulitic center depends on the polymer chain mobility. Thus, a factor in the spherulitic growth rate is the macroscopic viscosity of the polymer and its temperature dependence. All segmental motion is "frozen in" at the glass transition temperature (Tg) and no additional crystallization occurs even when nuclei are present. This Tg is about 50.degree. C. in nylon 6.
Other prior art describes other additives which can be used in addition to nucleating agents to enhance their effectiveness. For example, U.S. Pat. No. 3,549,651 discloses that the effectiveness of nucleating agents for linear polyamides in terms of the rate of crystallization into the desirable crystalline form can be increased by using a plasticizer in conjunction with the nucleating agent. U.S. Pat. No. 4,200,707 discloses the use of zinc phenyl phosphinate and zinc phenyl phosphonate as nucleating agents for certain polyamides.